Gavin Andresen- (Profile) Satoshi client maintainer. He previously worked at Silicon Graphics and now runs his own company.
Mark Karpeles aka MagicalTux- Owner of the largest bitcoin exchange, MtGox, this Bitcoin Wiki, and Kalyhost
Jeff Garzik- (Wikipedia Entry) Satoshi client core developer, Linux kernel developmer at Red Hat, and the founder of Bitcoin Watch.
Pieter Wuille aka sipa- Satoshi client developer and maintainer of the network graphs http://bitcoin.sipa.be
Stefan Thomas aka justmoon- creator of the WeUseCoins.com site/video and WebCoin.
Matt Corallo aka BlueMatt- Satoshi client developer.
Michael Marquardt aka theymos- creator of the widely used blockexplorer.com site, and BitcoinTalk Forum
Amir Taaki aka genjix- creator of the Britcoin and Intersango exchanges, libbitcoin, Spesmilo, Bitcoin Consultancy, Vibanko, GLBSE client, Bitcoin poker client and others.
Mike Hearn (Profile)- Google engineer who works on Gmail and developed BitCoinJ (http://code.google.com/p/bitcoinj/)
Luke Dashjr aka Luke-Jr- Eligius owner/admin and Spesmilo core developer
Vladimir Marchenko- Profile, runs Marchenko Ltd which sells mining contracts, previously developed the figator.org search engine.
Hal Finney- one of the creators of PGP and one of the earliest contributors to the bitcoin project.
Michael Hendrix aka mndrix- creator of the now defunct CoinPal and CoinCard services
Art Forz- developed the first GUI miner and at one time his GPU mining farm (the ArtFarm) was mining over a third of all blocks.
Sirius- Operates the host for bitcoin.org and is an administrator of the Bitcoin Forum.